Various embodiments relate to semiconductor devices and, more particularly, to semiconductor memory devices and methods of forming semiconductor memory devices.
Generally, semiconductor memory devices may be classified into volatile memory devices and nonvolatile memory devices. Volatile memory devices lose stored data when the power supply is interrupted. Examples of the volatile memory devices may include Dynamic Random Access Memories (DRAMs) and Static Random Access Memories (SRAMs). Meanwhile, the nonvolatile memory devices do not lose the stored data even when the power supply is interrupted. Examples of the nonvolatile memory devices may include Programmable ROMs (PROMs), Erasable PROMs (EPROMs), Electrically EPROMs (EEPROMs), and flash memory devices.
Developments have been actively made on next-generation semiconductor memory devices such as Ferroelectric Random Access Memories (FRAMs), Magnetic Random Access Memories (MRAMs), and Phase-change Random Access Memories (PRAMs) in accordance with the tendency toward improved performance and reduced power semiconductor memory devices. These next-generation semiconductor memory devices are made of materials that vary in programmed resistance state according to programming current and/or voltage and keep the programmed resistance state even though the power is interrupted.
A phase change memory device (PRAM) is a variable resistance memory device that utilizes a phase change material, and a phase change memory device (PRAM) may provide relatively high operation speed and high integration.
Phase change memory devices store information by using phase change materials. A phase change material has two stable states (that is, a crystalline state and an amorphous state) in which resistivity differs. Since the conversion may be reversibly generated between two stable states, the phase change material may be converted into the crystalline state from the amorphous state and may then be converted back into the amorphous state from the crystalline state. The phase change material may be converted into the amorphous state from the crystalline state and may then be converted back into the crystalline state from the amorphous state. The resistivity of the phase change material in the amorphous state is higher than that of the phase change material in the crystalline state. A phase change memory device can store data in phase change memory cells using the difference in resistivity according to the state of the phase change material and can read the data stored in the phase change memory cells.